1. Field of the Invention
The present invention relates to a drive system of a synchronous electrical motor.
2. Description of the Related Art
In an electrical motor drive system used in a vehicle, industry, a home electrical appliance, and the like, a small and highly efficient permanent magnet electrical motor (synchronous electrical motor) is widely used. Such a permanent magnet electrical motor is used in a torque assisted device, positioning control, fan, pump, compressor, and the like, for example.
In order to drive a permanent magnet electrical motor (abbreviated to PM electrical motor hereinafter), information about a rotation position of the PM electrical motor is required, and thus a position sensor therefor is required. In recent years, “sensorless control” in which a rotational speed or a torque of a PM electrical motor is controlled without such a position sensor is widely spread.
If the sensorless control can be practically used, cost expensed for a position sensor (cost of the sensor itself, cost expensed for wiring the sensor, and the like) can be reduced. In addition, due to the unnecessity of the sensor, the size of the system can be reduced and the system can be used in an adverse environment, thus causing a great advantage. Currently, for a sensorless control of a PM electrical motor, a method of directly detecting induced voltage (speed electromotive voltage) that is generated by rotation of the rotor, and driving a PM electrical motor by using the voltage as position information of the rotor, a position estimation technique for estimating and calculating a rotor position based on a mathematical model of an electrical motor to be controlled, and the like are used.
A big problem of these sensorless control schemes is a position detection method during low speed operation including the stopped state (zero speed). Most of sensorless controls that can be practically used currently are based on speed electromotive voltage generated by a PM electrical motor. Therefore, in a stop/low speed range where the induced voltage is small, the sensitivity is decreased and position information is buried in noise.
As a conventional scheme for solving this problem, JP-2009-189176-A discloses a control scheme for switching a current-applied phase based on induced voltage generated at an open phase in a position sensorless control scheme based on the 120 degree current application control of a PM electrical motor in a stop/low speed range. This induced voltage is induced voltage generated due to change of a magnetic circuit inside the PM electrical motor, and thus the scheme is on a principle different from that of the conventional scheme using speed electromotive voltage due to the rotation speed. Therefore, a sensorless control can be realized in the stop/slow speed range.
JP-2001-275387-A discloses a rotor position estimation technique using a position sensorless control scheme when the motor is stopped based on the 120 degree current application control of the PM electrical motor similarly to the method described in JP-2009-189176-A. In JP-2001-275387-A, when the PM electrical motor is in the stopped state, six voltage pulses are applied between phases of the three-phase windings (positive and negative pulses are applied between two phases out of the three phases), and based on the six induced voltage values generated at open phases at the respective time, the initial position of the rotor is estimated. With this method, the initial position of the rotor can be estimated in a short time, and thus a sensorless driving is realized without causing negative rotation of the electrical motor and also in a short time.